Particle size effect on structural and magnetic properties of Co0.75Ni0.25Cr2O4 composite nanoparticles

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This study explores the influence of particle size on the structural and magnetic characteristics of Co 0.75 Ni 0.25 Cr 2 O 4 composite nanoparticles prepared using the sol-gel method, with calcination temperatures varying between 500 °C and 900 °C at the intervals of 100 °C. The XRD results confirm that Co 0.75 Ni 0.25 Cr 2 O 4 has a cubic crystal structure with the space group Fd-3m . A secondary Cr 2 O 3 phase was detected in all samples that exhibits a rhombohedral structure with the space group R-3c . The particle sizes ranged from 7 ± 2 nm to 114 ± 7 nm, for 500 °C to 900 °C. Magnetization studies reveal size dependent magnetic transitions, with the Curie temperature, T C , ranging from 79 K to 89 K. Notably, all samples exhibited a spin-spiral transition temperature, T S , indicative of an incommensurate magnetic structure in nanocrystalline particles an unexpected result. Interestingly, T S increased varying between varying between 25 ± 1 and 48 ± 3 K, as particle size decreased. Furthermore, the experimentally determined effective magnetic moment for samples exceeded the theoretical estimate for Co 0.75 Ni 0.25 Cr 2 O 4 , highlighting the significant contribution of the orbital moments of Co 2+ and Ni 2+ ions to the overall magnetization. Field-dependent magnetization studies, M → ( μ 0 H → ) , confirmed ferrimagnetic behaviour in the samples, as evidenced by the presence of hysteresis below T C . The magnetization curve simulations effectively distinguished the ferrimagnetic, superparamagnetic, and paramagnetic components, revealing how the magnetic phase composition of Co 0.75 Ni 0.25 Cr 2 O 4 evolves with calcination temperature at 80 K. These findings demonstrate that nanoscale size reduction and Ni substitution significantly modify exchange interactions and spin ordering in CoCr 2 O 4 based spinel chromites.